The present invention relates, in general, to the field of linearized scanning systems and methods for an energy beam. More particularly, the present invention relates to a system and method for efficient utilization of a laser beam to provide localized zone melting for recrystallizing a thin silicon ribbon of use, for example, in the manufacture of solar cells.
In using focused laser beams to provide the localized melting needed for zone melting and recrystallizing a thin silicon ribbon, it is necessary to achieve a uniform distribution of laser power over a relatively long narrow zone. The narrow dimension of this zone is defined as the size of the focal spot of the laser beam and the longitudinal dimension by scanning the laser back and forth by means of an oscillating mirror driven by a driving signal having a triangular shaped waveform. Since the rotor of the scan galvanometer has a finite inertia, it follows that the available torque cannot make the mirror reverse direction instantaneously. By virtue of this fact, scan velocity is lower at the ends of the scan line thus producing higher temperatures at the ends of the melt zone.
One solution to the problem of providing uniform power density of a scan line would be to modulate the power of the laser itself without concern for the constant velocity of the scan. In other words, the power level of the laser could be reduced at the ends of the scan line to maintain relatively uniform temperatures along the melt zone. However, an inherent limitation in this approach is the fact that certain lasers are not readily modulated and therefore such a technique restricts the type of laser which might be employed. Moreover, such a modulation system is not readily effectuated and would require additional and unduly complex equipment.
Another approach to achieving essentially uniform power density along a scan line would be to chop the non-linear portion of each scan of the laser beam in synchronism with the scan galvanometer so as to eliminate these non-linear portions. This may be effectuated by mechanically interrupting the beam with a rotating disc. Utilizing this approach, the interrupted energy of the laser beam is simply lost.
In either of the above approaches, valuable laser capability is lost and a significant investment in laser capacity is not being efficiently utilized. In performing zone melting and recrystallizing of thin silicon ribbons, the capital cost of the laser used is a significant economic factor. For example, a typical high power laser costs on the order of approximately $100 per watt. Therefore, if a one kilowatt laser is being disabled 331/3% of the time, a sizable amount of energy, and hence capital investment is being wasted.